Numerical investigations on dynamic stall of low Reynolds number flow around oscillating airfoils

This paper presents a 2D computational investigation on the dynamic stall phenomenon associated with unsteady flow around the NACA0012 airfoil at low Reynolds number (Re_c ≈ 10⁵). Two sets of oscillating patterns with different frequencies, mean oscillating angles and amplitudes are numerically simulated using Computational Fluid Dynamics (CFD), and the results obtained are validated against the corresponding published experimental data. It is concluded that the CFD prediction captures well the vortex-shedding predominated flow structure which is experimentally obtained and the results quantitatively agree well with the experimental data, except when the blade is at a very high angle of attack.     

Innovative improvement of a drag wind turbine performance

Drag type wind turbines have strong potential in small and medium power applications due to their simple design. However, a major disadvantage of this design is the noticeable low conversion efficiency. Therefore, more research is required to improve the efficiency of this design. The present work introduces a novel design of a three-rotor Savonius turbine with rotors arranged in a triangular pattern. The performance of the new design is assessed by computational modeling of the flow around the three rotors. The 2D computational model is firstly applied to investigate the performance of a single rotor design to validate the model by comparison with experimental measurements. The model introduced an acceptable accuracy compared to the experimental measurements. The performance of the new design is then investigated using the same model. The results indicated that the new design performance has higher power coefficient compared with single rotor design. The peak power coefficient of the three rotor turbine is 44% higher than that of the single rotor design (relative increase). The improved performance is attributed to the favorable interaction between the rotors which accelerates the flow approaching the downstream rotors and generates higher turning moment in the direction of rotation of each rotor.     

Airfoil optimisation for vertical‐axis wind turbines with variable pitch

To advance the design of a multimegawatt vertical‐axis wind turbine (VAWT), application‐specific airfoils need to be developed. In this research, airfoils are tailored for a VAWT with variable pitch. A genetic algorithm is used to optimise the airfoil shape considering a balance between the aerodynamic and structural performance of airfoils. At rotor scale, the aerodynamic objective aims to create the required optimal loading while minimising losses. The structural objective focusses on maximising the bending stiffness. Three airfoils from the Pareto front are selected and analysed using the actuator cylinder model and a prescribed‐wake vortex code. The optimal pitch schedule is determined, and the loadings and power performance are studied for different tip‐speed ratios and solidities. The comparison of the optimised airfoils with similar airfoils from the first generation shows a significant improvement in performance, and this proves the necessity to properly select the airfoil shape.     

Comparative CFD analysis of Vertical Axis Wind Turbine in upright and tilted configuration

There is a demand of analyzing tilted Vertical Axis Wind Turbine (VAWT) for the emerging applications like VAWTs operating on high rise buildings, Floating Axis Wind Turbine (FAWT) functioning in deep sea, and so on. For this purpose numerical validation of an existing experimental work of Vertical Axis Wind Turbine (VAWT) in upright and tilted conditions has been carried out. The numerical validation is accomplished by means of Computational Fluid Dynamics (CFD) analysis by solving Unsteady Reynolds Averaged Navier–Stokes (URANS) equation. A rigorous parametric study of choosing a right turbulence model for solving URANS equation, mesh dependency analysis and optimum time step is comprehended. After choosing the right parameters, validation is performed. Following that the flow fields of upright and tilted conditions are visualized. The possible reasons of getting higher power coefficient in tilted condition have been discussed. While carrying out the study, it is observed that in case of tilted configuration the wake stream shifts downward. This feature of VAWT in tilted condition could lead into effective sea surface utilization in floating offshore wind farms.     

A Multiple Actuator Block model for vertical axis wind turbines

In this paper a new model to predict the wake of vertical axis wind turbines (VAWT) is proposed and analysed. The model is based on the actuator disk and the Double Multiple Stream Tube methods. Specifically, the model, denoted as Multiple Actuator Block, is based on the definition, inside the computational domain, of multiple parallelepipedic blocks distributed along the path of the blades. Volumetric momentum sinks are imposed in these blocks to model the effect of the blades on the flow. To analyse the performance of the model a VAWT with three NACA0022, for which numerical and experimental results are available in the literature, has been considered. Different types of simulations with the Multiple Actuator Block model have been carried out and have been compared with a complete finite volume simulation using the sliding mesh technique. This simulation requires about ten times more CPU time than the simulations using the Multiple Actuator Block model. It has been found that the large scale features of the far wake can be reproduced using the Multiple Actuator Block model applying in the block the forces, obtained from the complete finite volume simulation or obtained from a boundary-layer type code, when the blade is inside the block.     

椭圆积分方程的快速数值求解与误差分析

椭圆积分方程的查表求解效率较低而且容易引起数据误差。为此,文中应用数值方法对第一类完全椭圆积分和不完全椭圆积分的近似求解进行了研究,并将基于MATLAB完成的计算结果与数学用表进行了比对,分析了合理的设置计算步长对数值解法的结果误差的影响。结合应用实例,对给定k值的Troposkien啦线进行了坐标计算,同时计算了结果的理论误差值。     

ANALYTICAL AND GRAPHICAL METHODS OR DETERMINATION OF SOLAR CELL PARAMETERS AND INVESTIGATIONS OF SHADOWING EFFECT

In actual solar cells, the main power loss is due to the effect of the internal series resistance and the shunt resistance of the solar cell. Two methods; mathematical and graphical, were used to determine these two resistances for an Iraqi monocrystalline solar cell (type AI-Mansour). The results show that both of the series resistance (0·09 Ω) and the shunt resistance (210 Ω) can usually be neglected in an array performance evaluation for systems which don't use concentration arrangements

In addition to the series and shunt resistances computations, the analysis of the mismatching among solar cells as well as the power dissipation by shadowed or faulty cells for different module configurations are discussed in detail in this paper. As a result it was found that the maximum number of cells that can be safely series, parallel connected are 50 and 6 cells respectively.     

分布式能源用新型Senegal式垂直轴风力机尾流分析

针对现有的Jensen尾流模型分析水平轴风力机(HAWTs)的方法,对采用经典Jensen尾流模型时恒定的风速亏损和尾流衰减系数进行改进,提出基于改进型Jensen尾流模型对新型Senegal式垂直轴风力机(VAWTs)尾流分析的新方法。结合流场分析对该新型风力机进行尾流数值模拟,得出了基于改进的Jensen尾流模型的尾流参数二维分布的变化规律。由于尾流分布具有三维性,二维尾流分布不能完全反映尾流特性,因此还得到了尾流参数三维分布。全面地分析了单台VAWT的尾流特性,不仅为进一步的多台VAWTs尾流分析和风电场风力机排布研究打好基础,而且为该类Senegal式VAWT的尾流研究提供了借鉴和参考。     

A vortex panel model for the simulation of the wake flow past a vertical axis wind turbine in dynamic stall

A 2D vortex panel model with a viscous boundary layer formulation has been developed for the numerical simulation of a vertical axis wind turbine (VAWT), including the operation in dynamic stall. The model uses the ‘double wake’ concept to reproduce the main features of the unsteady separated flow, including the formation and shedding of strong vortical structures and the wake–blade interaction. The potential flow equations are solved together with the integral boundary layer equations by using a semi‐inverse iterative algorithm. A new criterion for the reattachment of the boundary layer during the downstroke of a dynamically stalled aerofoil is implemented. The model has been validated against experimental data of steady aerofoils and pitching aerofoils in dynamic stall at high and low Reynolds numbers (Re = 1.5 × 10⁶ and Re = 5 × 10⁴). For the low Reynolds number case, time‐resolved 2D particle image velocimetry (PIV) measurements have been performed on a pitching NACA 0012 aerofoil in dynamic stall. The PIV vorticity fields past the oscillating aerofoil are used to test the model capability of capturing the formation, growth and release of the strong leading edge vortex that characterizes the dynamic stall. Furthermore, the forces extracted from the PIV velocity fields are compared with the predicted ones for a quantitative validation of the model. Finally, the model is applied to the computation of the wake flow past a VAWT in dynamic stall; the predicted vorticity fields and forces are in good agreement with phase‐locked PIV data and CFD‐DES available in the literature. Copyright © 2012 John Wiley & Sons, Ltd.     

NACA00系列翼型的VAWT效能影响因素分析

采用滑移网格技术对不同参数下的小型垂直轴叶轮瞬态流场进行了数值计算,着重研究了不同安装角、翼型、叶片弦长、叶片数对叶轮功率的影响,得出不同转速下使叶轮获得最大功率的最佳翼型、叶片安装角、弦长和叶片数,并通过吹风实验验证了最佳叶片数和最佳弦长时的垂直轴风力机具有较优的气动性能。